1. Technical Field
The present embodiment relates to a stator including a stator core and a stator coil that generates a rotating magnetic field, and to an electric motor having this stator.
2. Description of Related Art
Conventionally, three-phase electric motors have been widely known. Such a three-phase electric motor has a stator and a rotor, with a rotating shaft installed at the center of the rotor. The rotating shaft is rotatably mounted on a housing through bearings, and rotates with the rotor as a current is applied to a stator coil. The stator has a stator core and the stator coil wound around the stator core, and the stator coil has three-phase coils, namely, a U-phase coil, a V-phase coil, and a W-phase coil. When a three-phase alternating current is applied to these three-phase coils, a rotating magnetic field is generated, causing the rotor to rotate.
The conventional electric motors are afflicted with a problem that electrical corrosion occurs on the bearings supporting the rotating shaft. This will be described with reference to FIG. 16. FIG. 16 is a view showing the configuration of a conventional electric motor 10. Any magnetic unbalance occurring inside the electric motor 10 results in a magnetic flux of higher electrical frequency (hereinafter referred to as an unbalanced magnetic flux 50) being generated around a rotating shaft 16. Then, a voltage (hereinafter referred to as a shaft voltage) is induced across both ends of the rotating shaft 16 due to the unbalanced magnetic flux 50. FIG. 17 is a graph showing one example of waveforms representing a shaft voltage VS and a current flowing through a U-phase coil (U-phase current AU). As shown in FIG. 17, the shaft voltage VS is a tertiary harmonic voltage having a frequency three times higher than the fundamental frequency. The shaft voltage VS is applied through the rotating shaft 16 and a housing 18 to the inner and outer rings of rotating bearings 19. Although the inner and outer rings of the bearing 19 are insulated from each other with a lubricating oil film, this lubricating oil film is as thin as several μm, and thus insulation breakdown occurs when a voltage above a certain threshold (about several volts) is applied. Once the insulation between the inner and outer rings of the bearing 19 has broken down, an induced current 52 flows through a circulation route from the rotating shaft 16 to the bearing 19 to the housing 18 and back to the rotating shaft 16 as indicated by the broken line in FIG. 16. A problem arises here that the joule loss concentrates at the part of insulation breakdown, i.e., the bearing 19, which promotes electrical corrosion of the bearing 19.
To suppress such electrical corrosion of the bearings, the technique disclosed in Japanese Patent Application Publication No. 2014-11827 involves separately providing a conductive member that mechanically couples together a rotating shaft and a housing. This configuration can suppress electrical corrosion of the bearings by causing an induced current to flow dominantly to the conductive member that has lower impedance than the bearings.